(1) Field of the Invention
This invention relates to an apparatus and method for image processing by which two images are superimposed one on the top of the other, and to an image forming apparatus, such as a copying machine, equipped with such an apparatus for image processing.
(2) Description of the Related Art
Various apparatuses for superimposing two images one on the top of the other have been known, one of which is a digital-erasing apparatus, used in color copying machines and the like to erase analog images. According to this apparatus, a document (an analog image) is reflected onto the surface of a photoconductor through an optical system consisting of an exposure lamp, mirrors, and a lens in order to form an electrostatic latent image. The undesired area of the document is erased by use of a light emitting diode (hereinafter referred to as LED) head.
The above-mentioned digital-erasing apparatus, which supplies black toner to black electrostatic latent images formed on the photoconductive drum, is suitably used to obtain preferable black images because black made by composing yellow, magenta, and cyanine is not natural black. The LED head successively erases an area of each electrostatic latent image of yellow, magenta, and cyanine, the area turning out to be black when these three color images are superimposed altogether. Lighting signals corresponding to the black area detected on a document read by an image sensor, or a CCD are supplied line by line to the LED head synchronously with latch pulses controlling the supply of the light signals. Generally, such latch pulses have a certain cycle period; however, the torque fluctuation or the like during the scanning of the optical system may cause a scanner motor used to form analog images to be driven at an irregular speed. This brings about a problem that an analog image and a digital image formed based on digital data so as to erase part of the analog image may not coincide both in position and size.
If the scanning speed is two-thirds as low as the regulation speed, an analog image Ib shown in FIG. 16 (b) is formed which is 1.5 as long in the sub scanning direction as the expected black image Ia shown in FIG. 16 (a). This is because the scanning speed is low while the rotating speed of the photoconductor is regulated. As shown in FIG. 16 (c), a digital image Ic corresponds in shape to the analog image Ia because the cycle period of the latch pulses is regulated. When both images Ic and Ib are superimposed one on the top of the other on the photoconductor in order to erase the analog image Ib, they appear as shown in FIG. 16 (d). Thus, the irregular scanning speed makes magnification of an analog image and a digital image disaccord in the sub scanning direction. Furthermore, in most cases, even the record starting points of the images Ib and Ic do not coincide, which prevent the analog image from being preferably erased.